The demand for clean drinking water continues to surge, resulting in an increasing demand for materials and strategies for remediation of contaminated aqueous systems. Clean drinking water is a health concern for the entire population, and as such, is a focal point for the World Health Organization (WHO). Sorbent technologies, like functionalized silica substrates, are prime candidates for water remediation and analytical separations due to their ability to selectively remove harmful contaminants at trace levels. These materials also hold promise for uranium mining from seawater, collection of desired metals from mining streams, and trace collection of specific contaminants in industrial aqueous waste streams. The growth in research in mesoporous silicas and the breadth of application of these materials have expanded greatly over the last twenty years. Functionalized silicas are also applicable in fields such as biomedical engineering, supported catalysis, and molecular motors. The high specific surface area, absorption capacity, robust nature, and tunability of the surface chemistry make functionalized silica substrates particularly well suited as sorbents. Many of these properties are governed by the substrate, but the affinity of the material for a target analyte is dependent on the ligand used for functionalization.
A material's surface dictates the way in which it interacts with its surroundings. The effectiveness of sorbent materials to remove trace organic and inorganic contaminants is dependent upon the ability to install high affinity chelating ligands with high density coverage. The greater the selectivity that the chelating ligand exhibits for the contaminant, the more effective the sorbent will be. To achieve desired affinity and selectivity, it is often necessary to attach somewhat delicate functional groups to material surfaces. Unfortunately, many of the established surface functionalization techniques have suffered from poor ligand loading and attachment with these functional groups. Challenges in the installation of surface chemistries on sorbent supports include long reaction times, steric hindrance from protecting groups, and complete deprotection of the reactive moiety. Steric hindrance leads to a lowered density of ligand loading, while the deprotection and activation of the desired surface chemistry often does not go to completion, further lessening the material's performance. In addition, the deprotection methods commonly used can have detrimental effects on the support materials.
Rare earth elements (REEs) are used extensively in a variety of modern technologies including electronic devices, permanent magnets, automobile catalysts, metallurgical additives, and glass/ceramic additives and polishing. The REEs are extensively produced and purified in three countries: China, USA, and India—with over 80% of world's REE resources presently coming from the mining in China. Due to their importance, the supply of REEs has received increasing attention during this decade, especially for clean energy applications and electronic devices, such as smart phones and computers. This has fueled research and development to improve the collection and recycling of REEs through different techniques from various resources, including natural waters, geothermal fluids, and waste streams of electronic and nuclear wastes. The natural waters (ground, river and sea waters) have been found to contain trace amount of REEs, which are reported to have come partly from mining drainages and waste discharges. While geothermal fluids naturally contain dissolved REEs, their concentration and presence depends on location, source rocks, and temperature.
However, in practice, the recovery and recycling of REEs and precious metals from aqueous resources are still very challenging. This is due to the very low levels of these metals contained in natural waters, geothermal fluids, and waste waters, coupled with the limitations of collection and separation technology. These reasons, combined with the economic demand and recent REE price, show that improved technology for recovery and recycling are needed in the near future.
Solid phase sorption is one of the most extensive and effective techniques for the removal of trace metals from aqueous solutions. It offers a number of advantages, such as flexible configurations, easy application and operation, low waste generation, and the ability to be scaled up and regenerated, resulting in a potential economical solution to REE recycling. The sorption efficiency is dependent on surface area, surface chemistry, and active site density of the sorbent. Functionalized sorbents can be designed, tailored, and synthesized with selected attractive surface chemistries for enhancing the sorption of the trace metals of interests. Several typical sorbents with differing surface chemistries have been developed and applied for REE sorption including metal oxides, ion exchange, chelating and functional group complexation. Among these, the phosphonic acid moieties are the most frequently studied.
Recovering REEs from natural waters using sorbents becomes more economically feasible as the rate of collection, and the concentration of dissolved desired metals increases. The rate of collection is dependent on the concentration of dissolved REEs, the selectivity and affinity of the sorbent, and the water flow rate. There are trace concentrations of dissolved REEs in natural waters, but in geothermal waters and mining discharges the concentrations have been found to be significantly increased. The sorbent materials need to remove the maximum possible amount of REEs, and ideally would be selective to avoid flooding the materials with unwanted species